This section introduces aspects that may be helpful to facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light. The statements of this section are not to be understood as admissions about what is in the prior art or what is not in the prior art.
There is great interest in the use of engineered surfaces to reduce the flow resistance of a liquid on the surface. Some structured surfaces having nanometer- or micron-sized raised features have promise in applications ranging from the transport of a liquid through a channel, to reducing the drag of a vessel traveling through a liquid. However, problems must be overcome before the full benefit of these surfaces can be realized.
One problem is that the flow resistance of a liquid on a structured surface can vary dramatically with the pressure of the liquid. If the pressure of the liquid increases, then the liquid will penetrate to a greater extent into the structured surface, thereby increasing the flow resistance of the liquid on that surface. Alternatively, if the pressure of the liquid decreases, then the liquid will penetrate to a lesser extent into the structured surface, thereby decreasing the flow resistance. Flow resistance can also increase when the diffusion of air out of the liquid is sufficient to form air bubbles on the structured surface. For instance, the formation of air bubbles on a structured surface that is covering the inner surface of a pipe or channel can significantly increase the flow resistance of a liquid by partially blocking the pipe or channel cross-section.
Embodiments of the present invention overcome these deficiencies by providing an apparatus having a structured surface that facilitates formation of a foam that provides improved pressure stability and reduced flow resistance, as well as methods of using and manufacturing such an apparatus.